This application claims the benefit of a Japanese Patent Application No. 2002-348789 filed Nov. 29, 2002, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to power supply control methods, current-to-voltage conversion circuits and electronic apparatuses, and more particularly to a power supply control method for making a standby power zero, a current-to-voltage conversion circuit for use by such a power supply control method, and an electronic apparatus which uses such a current-to-voltage conversion circuit.
A current-to-voltage conversion circuit (hereinafter simply referred to as current-voltage conversion circuit) or, a power supply circuit, which is used in an AC adapter or the like, converts a commercial AC power supply voltage into a DC power supply voltage which is required by an electronic apparatus. The current-voltage conversion circuit consumes power even when the electronic apparatus is in a standby state or a stopped state, and such a consumed power is often referred to as a standby power. The standby power is generated even if the power consumption of the electronic apparatus is zero, because of an excitation power consumed by a magnetic circuit, such as a transformer, which is assembled within the current-voltage conversion circuit.
2. Description of the Related Art
In portable electronic apparatuses such as lap-top personal computers, a battery is provided as a power supply for the electronic apparatus. Generally, for reasons such as the operating cost of the electronic apparatus and an instantaneously dischargeable current capacity, a secondary battery such as a Li+ (lithium ion) battery is provided. In addition, a charging circuit is provided in most electronic apparatuses, so that the secondary battery of the electronic apparatus may be easily charged by connecting an AC adapter or the like to the electronic apparatus.
In the case of the portable electronic apparatus, the secondary battery is normally used as the power supply of the electronic apparatus. But when operating the portable electronic apparatus on a desk, for example, the electronic apparatus may be operated by using an external power supply via the AC adapter. When the electronic apparatus is operated by the external power supply via the AC adapter, the AC adapter operates to output a rated voltage, even when the electronic apparatus is in the standby state or the stopped state.
FIG. 1 is a circuit diagram showing an example of a conventional AC adapter which converts a commercial AC power supply voltage into a DC power supply voltage which is required by an electronic apparatus. The AC adapter shown in FIG. 1 includes a rectifying circuit 1 for rectifying the commercial AC power supply voltage, a voltage conversion circuit 2 for converting an input voltage into an output voltage, a rectifying circuit 3 for rectifying a secondary side output of a transformer T1 within the voltage conversion circuit 2, an output control circuit 4 for controlling the secondary side output, and a coupler circuit 5 for transferring a control state of the secondary side output to a primary side of the transformer T1 within the voltage conversion circuit 2.
The rectifying circuit 1 includes rectifying diodes D1 through D4 for subjecting an AC input to a full-wave rectification, and a smoothing capacitor C1 for smoothing a rectified input thereto. The voltage conversion circuit 2 includes the transformer T1 for voltage conversion, a switching circuit FET1 for turning ON/FF a current which flows through the transformer T1, and a drive control circuit 21 for controlling an ON/OFF state of the switching circuit FET1. The rectifying circuit 3 includes a rectifying diode D5 for rectifying a voltage which has been converted by the voltage conversion circuit 2, and a smoothing capacitor C2 for smoothing a rectified input thereto.
The output control circuit 4 includes a sense resistor R0 for detecting an output current, and a control circuit 41 for controlling the output current and the output voltage. The coupler circuit 5 transfers the output of the output control circuit 4 to the primary side of the transformer T1. For example, the coupler circuit 5 is formed by a photo-coupler which electrically insulates the primary side and the secondary side of the transformer T1.
FIG. 2 is a circuit diagram showing the control circuit 41 and the drive control circuit 21 shown in FIG. 1. In FIG. 2, the drive control circuit 21 includes a triangular wave oscillator 22, a pulse-width-modulation (PWM) comparator 23, and a drive circuit 24. The control circuit 41 includes a voltage amplifier AMP11, error amplifiers ERA11 and ERA12, transistors Tr11 and Tr12, and a current source 42.
In FIG. 2, a reference voltage e11 determines the output current value, and a reference voltage e12 determines the output voltage value. The voltage amplifier AMP11 of the control circuit 41 measures a voltage drop caused by a current flowing through the sense resistor R0, and outputs a voltage proportional to the current value flowing through the sense resistor R0. The error amplifier ERA11 compares the output voltage of the voltage amplifier AMP11 and the reference voltage e1. If the current flowing through the sense resistor R0 is large, a small voltage is output from the error amplifier ERA11. A large voltage is output from the error amplifier ERA11 if the current flowing through the sense resistor R0 is small. Similarly, the error amplifier ERA12 compares an output voltage of the AC adapter and the reference voltage e2.
The transistors Tr11 and Tr12 form a circuit for outputting a smaller one of output voltages of the error amplifiers ERA11 and ERA12. The smaller one of the output voltages of the error amplifiers ERA11 and ERA12 is supplied to the PWM comparator 23 of the drive control circuit 21, via the coupler circuit 5 which electrically insulates the primary side and the secondary side of the transformer T1.
The PWM comparator 23 within the drive control circuit 21 has a non-inverting input terminal and an inverting input terminal, and is a kind of a voltage pulse width converter which controls an ON-time of an output pulse depending on the input voltage thereto. The PWM comparator 23 outputs a signal which becomes ON during a time in which the triangular wave input to the inverting input terminal from the triangular wave oscillator 22 is smaller than the output voltage of the control circuit 41 which is input to the non-inverting input terminal via the coupler circuit 6. The output signal of the PWM comparator 23 is output to the drive control circuit 21 via the drive circuit 24.
In FIG. 1, when the switching circuit FET1 is ON, the input current from the rectifying circuit 1 flows to the primary side coil of the transformer T1, and the output current flows to the secondary side coil of the transformer T1 when the switching circuit FET1 is turned OFF. An energy stored in the primary side coil of the transformer T1 and an energy discharged from the secondary side coil of the transformer T1 are the same, and thus, an output voltage Vout may be obtained from the following formula (1), where Vin denotes the input voltage, Ton and Toff respectively denote the ON-time and the OFF-time of the switching circuit FET1, and it is assumed for the sake of convenience that a number of turns of the primary side coil of the transformer T1 and a number of turns of the secondary side coil of the transformer T1 are the same.Vin×Ton=Vout×Toff  (1)
Accordingly, if the formula (1) is rearranged for the output voltage Vout, the following formula (2) is obtained, and a change in the input voltage Vin can be controlled by a ratio of the ON-time Ton and the OFF-time Toff of the switching circuit FET1.Vout=(Ton/Toff)×Vin  (2)
The AC adapter operates to always output the rated voltage when the AC power supply voltage is input thereto. Hence, the AC adapter operates to always output the rated voltage as long as the AC adapter is connected to the commercial AC power supply, regardless of whether or not the AC adapter is connected to the electronic apparatus. For this reason, even if the electronic apparatus which is connected to the AC adapter is in a power supply OFF state and consumes no power, and the AC adapter is in a no-load state, the AC adapter still operates to output the rated voltage.
Therefore, even in the no-load state of the AC adapter, the control circuits 21 and 41 within the AC adapter operate to output the rated voltage, and the AC adapter itself consumes the standby power. In order to prevent the AC adapter from consuming the standby power, it is necessary to completely stop the operation of the AC adapter, but in order to be able to start the operation of the electronic apparatus, which is connected to the AC adapter, at any time, the AC adapter must always be in the standby state.
Various methods have been proposed to reduce the standby power of the AC adapter when the electronic apparatus is in the standby state or the stopped state.
According to a first conventional method, the operating speed or frequency of the AC adapter is decreased or, the operating frequency of the AC adapter is decreased while at the same time intermittently operating the AC adapter, so as to reduce the power consumption of the AC adapter itself while maintaining the desired output voltage. More particularly, the operating frequency of the AC adapter is decreased by decreasing an oscillation frequency of the triangular wave oscillator shown in FIG. 2. This first conventional method is proposed in a Japanese Laid-Open Application No. 2000-217161, for example.
According to a second conventional method, the power consumption of the primary side circuit of the AC adapter is reduced. In other words, since the primary side of the AC adapter operates by the commercial AC power supply voltage, the power consumption is reduce by decreasing the AC voltage. When the AC adapter starts to operate, the operation is started using the primary side input voltage. However, after the operation of the AC adapter starts, the power consumption of the AC adapter is reduced by utilizing a third voltage which is created in the AC adapter and is lower than the primary side input voltage.
FIG. 3 is a circuit diagram for explaining the second conventional method. In FIG. 3, those parts which are the same as those corresponding parts in FIG. 1 are designated by the same reference numerals, and a description thereof will be omitted.
As shown in FIG. 3, a switching circuit FET2, a diode D6, and a third winding L3 of the transformer T1 are provided in a voltage conversion circuit 2-1. The switching circuit FET2 is provided to turn ON/OFF the supply of the commercial AC power supply voltage with respect to the AC adapter. This switching circuit FET2 is turned ON/OFF by a primary side drive control circuit 21. The third winding L3 of the transformer T1 is provided to generate the third voltage by the transformer T1. The diode D6 is provided to rectify the third voltage generated by the third winding L3.
When the commercial AC power supply voltage is supplied to the AC adapter, the commercial AC power supply voltage is applied to the AC adapter via the switching circuit FET2, and the AC adapter starts to operate. When the AC adapter operates and the rated voltage is output on the secondary side of the transformer T1, a voltage is also output at the third winding L3 of the transformer T1. The primary side drive control circuit 21 turns OFF the switching circuit FET2 after the operation of the AC adapter starts, so as to switch the voltage supplied to the primary side drive control circuit to the third voltage which is generated by the third winding L3 which is added to the transformer T1. This third voltage is sufficiently lower than the commercial AC power supply voltage. Hence, the power consumption of the AC adapter is reduce by decreasing the voltage which is supplied to the primary side drive control circuit 21.
According to a third conventional method, two systems of AC-DC current-voltage conversion circuits are provided, and the AC-DC current-voltage conversion circuits which are to operate are switched depending on whether the electronic apparatus is in the operating stage or the standby state. This third conventional method is proposed in a Japanese Laid-Open Patent Application No. 2001-145355, for example.
According to the first through third conventional methods, a part within the current-voltage conversion circuit is always operating, even when the electronic apparatus which is connected to the current-voltage conversion circuit, such as the AC adapter, is in the standby state or the stopped state. For this reason, there was a problem in that it is impossible to reduce the power consumption of the current-voltage conversion circuit to zero, that is, to reduce the standby power to zero.